In foil blowing processes, generally an extruded tube of a thermoplastic synthetic resin is blown into a tubular balloon (blown tube) which is flattened at an upper end into a flattened tube consisting of two webs of the synthetic resin foil, joined together at edge folds.
The apparatus can include a pair of squeezing rolls above the foil blowing part of the apparatus which closes off the top of the balloon and allows the air within the balloon to stretch the extruded synthetic resin tube to thin the latter, thereby producing the foil.
An apparatus of this type can include an extruder which is provided at the bottom of the tube with a foil blowing head. In an apparatus of the type with which the invention is concerned, the blowing head does not rotate, either continuously or in a reversible manner. The extruder unit is also stationary. The blown-foil tube has a circumference 2 .pi.r where r is the diameter of the blown balloon as measured at an intermediate section of the height thereof between the blowing head and the squeezing rolls at the lower and upper ends of the blown foil tube (see German Patent Document 26 39 551 C2).
Upon the flattening of the blown foil tube, two foil webs are formed, each of width .pi.r, joined along the edges of the flattened tube by edge folds.
To avoid the build up of barrel-shaped bulges or singular defects in the roll in which the flattened foil tube is wound, it is known to shift the folds along the periphery of the tube by fold-shifting means downstream of the blowing device. The fold shifting can be effected periodically with a corresponding reversing drive of the flattening device.
By angularly displacing the flattening unit, systematic defects are spread over the length of the roll or coil in which the flattened tube is wound to avoid the development of a barrel shape or bulges or the like in that coil. In the past, the flattening device generally had to be rotated through 360.degree.. The complexity of such a drive and the complicated kinematics of the mechanism required to accomplish it were generally pronounced. A lesser displacement, however, with the earlier systems already described, could not fully distribute the defects over the length of the roll.
In EP 0 408 996 A2, the fold-shifting system is located along the central axis of the flattening/squeezing device and comprises two rods or bars disposed above one another and respective deflecting rollers cooperate with these bars. The flattened tube arrives at and leaves each deflecting roller in a direction perpendicular thereto and at least at a starting point the flattened tube arrives at a turning bar at an angle of 45.degree. and leaves the turning bar at an angle of 45.degree. so that the angle between arriving and departing stretches at each turning bar is 90.degree.. During the swinging movement, this angle diminishes although the maximum angle is periodically restored.
With this construction as well the kinematics are complex and the control system required may not be fully reliable.
In fact, while the system generally operates well for small diameter blown foil tubes, with extremely large diameter blown foil tubes and very wide webs, the uniform passage of the flattened tube over the bar cannot be guaranteed. As a consequence, creases or the like may develop which defeat the entire purpose of the fold-shifting unit.